The technology pertains to telecommunications, and particularly, to a frame structure and a method and apparatus for dynamically configuring a frame structure.
In a typical cellular radio system, radio or wireless terminals (also known as mobile stations, user equipment units (UEs), UE radio terminals, UE terminals, etc.) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS), “eNodeB” (LTE), or more generally a radio network node. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Release 10 for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification has issued recently, and as with most specification, the standard is likely to evolve. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE). As such, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
Transmission and reception from a node, e.g., a radio terminal like a UE in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). In Frequency Division Duplex (FDD), as illustrated to the left in FIG. 1, downlink and uplink transmission take place in different, sufficiently separated, frequency bands. In Time Division Duplex (TDD), as illustrated to the right in FIG. 1, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired frequency spectrum, whereas FDD requires paired frequency spectrum.
Typically, a transmitted signal in a communication system is organized in some form of frame structure. For example, LTE uses ten equally-sized subframes 0-9 of length 1 msec per radio frame as illustrated in FIG. 2.
In the case of FDD operation (illustrated in the upper part of FIG. 2), there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). At least with respect to the radio terminal in a cellular communication system, FDD can be either full duplex or half duplex. In the full duplex case, a terminal can transmit and receive simultaneously, while in half-duplex operation (see FIG. 1), the terminal cannot transmit and receive simultaneously (although the base station is capable of simultaneous reception/transmission, i.e., receiving from one terminal while simultaneously transmitting to another terminal). In LTE, a half-duplex radio terminal monitors/receives in the downlink except when explicitly instructed to transmit in the uplink in a certain subframe. FDD half-duplex operation is specified in the 3GPP MAC specification TS 36.321.
In the case of TDD operation (illustrated in the lower part of FIG. 2), there is only a single carrier frequency, and uplink and downlink transmissions are separated in time. Because the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither downlink nor uplink transmissions occur in order to avoid interference between uplink and downlink transmissions. For LTE, “special” subframes (subframe 1 and, in some cases, subframe 6) provide this guard time. A TDD special subframe is split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission. The DwPTS part of the special subframe is used for Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH) transmission, whereas the UpPTS part is used only for random access preamble transmission on Physical Random Access Channel (PRACH), and for sounding, i.e., sounding reference signals (SRS).
Time division duplex (TDD) allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations as shown in FIG. 3. The configurations cover a wide range of allocations from an uplink heavy DL:UL ratio 2:3 (Configuration 0) to a downlink heavy DL:UL ratio 9:1 (Configuration 5). These configurations are referred to in examples below.
The required length of the guard period depends on the network deployment, cell sizes, etc. As a result, 3GPP specifies 11 different special subframe configurations. The special subframe configuration defines how 14 symbols of the subframe are divided between the downlink part (DwPTS), the uplink part (UpPTS), and the guard period (GP). The specified special subframe configurations are depicted in Table 1 below. The TDD configuration as well as the 3GPP “special” subframe configuration are signaled to the UEs on a broadcast channel as a part of System Information Block 1. In a handover scenario, use of dedicated radio resource control (RRC) signaling is also possible.
TABLE 1GuardConfigurationDwPTSperiodUpPTS031011941210313112141211539269327102281112
To avoid significant interference between downlink and uplink transmissions between different cells, neighbor cells should have the same downlink/uplink configuration. Otherwise, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell (and vice versa) as illustrated in FIG. 4, where the uplink transmission of the UE in the right cell is interfering with the downlink reception by the UE in the left cell. As a result, the downlink/uplink asymmetry can typically not vary between cells. However, there might some special deployments, e.g., with isolated cells, where the interference is not a problem, and thus, a different TDD configuration in neighboring cells is possible. But the downlink/uplink asymmetry configuration signaled as part of the system information remains fixed for a long period of time with existing mechanisms.
Existing TDD networks typically use a fixed configuration where some subframes are uplink and some are downlink. This limits the flexibility in adopting the uplink/downlink asymmetry to varying traffic situations.
One possibility to increase the flexibility of a TDD system, at least in some scenarios, is disclosed in commonly-assigned U.S. patent application Ser. No. 12/816,821 and summarized here. Each subframe (or part of a subframe) belongs to one of three different types: downlink, uplink, and a new type called “flexible.” A downlink subframe is used (among other things) for transmission of downlink data, system information, control signaling, and hybrid-ARQ feedback in response to uplink transmission activity. For example, in LTE Rel-8, the UE monitors the physical dedicated control channel (PDCCH) subframes for scheduling assignments and scheduling grants. Uplink subframes are used (among other things) for transmission of uplink data, uplink control signaling (e.g., channel-status reports), and hybrid-ARQ feedback in response to downlink data transmission activity. For example, in LTE Rel-8, data transmission on the physical uplink shared channel (PUSCH) in uplink subframes is controlled by uplink scheduling grants received on a PDCCH in an earlier downlink subframe. “Flexible” subframes described in the commonly-assigned U.S. patent application Ser. No. 12/816,821, which are not 3GPP “special” subframes, may be used for uplink or downlink transmissions. The flexible subframe is used to select a particular communication direction (uplink or downlink).
In the commonly-assigned U.S. patent application Ser. No. 12/816,821, a semi-static configuration is used to assign one of the above three types to each subframe as illustrated in FIG. 5. For example, semi-static configuration means, in a non-limiting LTE context, configuration by a medium access control (MAC) control element (CE), RRC, or a specific radio network temporary identifier (RNTI) on a PDCCH, and may for example be part of the system information either by explicitly indicating “UL”, “DL”, or “flexible,” or by signaling “DL” and “UL” using an existing signaling message and then introduce an additional signaling message, understandable by new radio UE terminals only, where some subframes are identified as flexible. From a UE perspective, flexible subframes may be treated in a similar way as DL subframes unless the UE has been instructed to transmit in a particular flexible subframe. In other words, flexible subframes not assigned for uplink transmission from a particular UE may be treated as a DL subframe. If the control signaling contains an uplink scheduling grant valid for a later subframe, then the UE will transmit in the uplink using one or more flexible subframes.
In addition to downlink assignments and uplink scheduling grants, other types of control signaling should be considered. Of particular interest are hybrid-ARQ (HARQ) acknowledgement messages (could be positive or negative) transmitted in one direction in response to data transmission in the other direction. As an example, when the UE in LTE receives a data transmission in a particular subframe from the eNodeB, it will, at a predetermined time, transmit a hybrid-ARQ acknowledgement informing the eNodeB whether the data transmission was successful or not. Commonly-assigned U.S. patent application Ser. No. 12/816,821 proposes to transmit feedback signaling only in an uplink or downlink subframe and not in flexible subframes.
Dynamic TDD is further extended in commonly-assigned U.S. patent application Ser. No. 12/945,554, filed on Nov. 12, 2010, the contents of which are incorporated herein by reference. Here, the UE uses primary and secondary TDD configurations to determine if the subframe is a DL, UL, or flexible subframe. FIG. 5 illustrates a non-limiting example radio frame that includes downlink, uplink, and flexible subframes. The primary TDD configuration may have more UL subframes than the secondary TDD configuration. U.S. patent application Ser. No. 12/945,554 also proposes in one embodiment that the timing of UL HARQ follows one TDD configuration (primary configuration) with the timing of DL HARQ following another configuration (secondary configuration). FIG. 6 is a non-limiting example illustrating HARQ feedback timing according to a secondary TDD configuration compared to a primary TDD configuration. UEs that do not support dynamic TDD must follow the primary TDD configuration because in this configuration, all flexible subframes are UL subframes, and a subframe cannot dynamically change from DL to UL direction, which would disturb reception of reference signals.
A desirable goal is to permit legacy UEs to continue to access radio networks that include technical features that the legacy UEs are not configured to utilize. For example, legacy UEs should be able to receive reference symbols and make measurements in any subframe that these legacy UEs consider downlink subframes. If the network omits transmission of reference symbols during a particular flexible downlink subframe, and the legacy UEs are not aware of this, the result can be disturbed link quality measurements and potentially unnecessary handovers to neighboring cells.
In the above-referenced patent applications, backwards compatibility is achieved by configuring legacy UEs with the legacy TDD configuration in such a way that all flexible subframes are UL subframes. In this way, legacy UE measurements are not disturbed because reference symbols (RS) will not be transmitted in UL subframes. Because legacy UEs can be configured only with legacy TDD configurations (presented in FIG. 3), the maximum amount of UL resources that can be allocated in the above example using dynamic TDD in this context corresponds to TDD Configuration 0 with 6 out of 10 subframes. But in some cases, a large amount of UL resources may be desirable to offload uplink traffic spikes. Furthermore, UEs in a bad coverage area may be power-limited making it difficult even sustain low bitrates. UEs in this situation would benefit from a larger allocation of UL subframes because the available uplink transmission time is longer.
In current 3GPP specifications, a “special” subframe configuration is signaled to UEs in a semi-static manner in broadcasted System Information. Although there is a need to be able to change the guard period of the special subframe dynamically, even within a radio frame, it currently is not possible to adapt the special subframe configuration dynamically. This means that the guard period needs to be semi-statically dimensioned to long enough to accommodate a worst scenario, resulting capacity loss, and when a change is needed, there is an associated delay due to the need for reconfiguration.
One example where the needed guard period may change rapidly is when there is interference from a remote base station (BS) via an atmospheric duct. The presence of such interference is often time-varying and may require an increased guard period. In the 3GPP specifications today, the guard period can be increased by decreasing the duration of the downlink part of the special subframe, DwPTS, but the challenge with this approach is the need to change the guard period not only in the local base station that suffers from the interference (i.e., DL/UL radio channel reciprocity causes interference to the remote base station) but also in the remote base station. A problem in this situation is that it is typically not known which remote base station is causing the interference, and even though the radio channel itself is reciprocal in the uplink and downlink directions, the interference is not necessarily reciprocal because it depends on traffic load and on transmit power.
An alternative approach that can be implemented in a distributed fashion is to increase the guard period by effectively shortening the uplink period. For uplink data transmission, this shortening can be accomplished by not granting uplink transmissions in the first uplink subframe after a guard period. But due to predefined timing relations between the uplink and downlink, uplink control signaling can not be removed from the first uplink subframe. This also means that when there is interference in the uplink, the downlink performance is also adversely affected since control information such as HARQ ACK associated with one or typically several downlink subframes are transmitted in the first subframe.
The technology in this application solves these and other problems.